The present invention relates to an improved lead acid battery element containing metal impurity inhibiting polymeric additives which are added to the positive active material, negative active material and/or battery separator to inhibit the detrimental effects of certain metals on the efficiency of a lead acid battery, particularly the negative plate battery element and to macroporous additives that enhance active material utilization efficiency and improvement in the utilization of sulfuric acid electrolyte necessary for the discharge reaction of a lead acid battery.
Metal impurities can be introduced into a lead acid battery through the use of any of the materials used in the manufacture of the battery. For example, metal impurities can be introduced in the lead and leady oxides used in the manufacture of the active material, the materials of construction including the lead grids, alloying agents, electrolyte and water. Nearly all metallic impurities, if they are nobler than lead, have a smaller hydrogen overvoltage than pure lead. Therefore, they increase hydrogen evolution even if they are deposited in minute concentrations on the surface of the negative plates. These metals cause a continued gas evolution even after charging is completed. Hydrogen is evolved on the deposited metal with low hydrogen overvoltage, which can be short-circuited with lead. The effect of metal on the gassing particularly postcharge gassing decreases in the following sequence: Pt, Au, Te, Ni, Co, Fe, Cu, Sb, Ag, Bi and Sn. The presence of 0.3 ppm of platinum in the acid can cause a doubling of the self-discharge rate. Tin can produce this effect at 0.1%. Freshly deposited antimony is especially active. Besides the discharge of the negative plates with concomitant hydrogen evolution, these materials also move the end of charge voltage of the negative plates toward more positive values. The hydrogen evolution increases with rising acid density. Because the hydrogen overvoltage decreases with temperature, the self-discharge increases.
In addition, antimony is often added to grid lead in order to make the lead more fluid and more easily cast into the shapes necessary for storage battery grids. Further, it also hardens the resulting casting so that it can be further processed in the plant without damage. In certain battery applications, it may be necessary for the battery to withstand extreme resistance to corrosion of positive plate grids. In that event, higher antimony contents typically within the range of 4.5 to 6 percent are incorporated into the grid to form a lead antimony alloy. Antimony in these concentrations are generally only used in positive grids particularly grids intended for corrosion resistant batteries. Corrosion resistance typically means the ability to withstand the destructive effects of excessive charge or overcharge.
Antimony in the grid metal produces a definite effect on the charge voltage characteristics of the fully charged wet battery. The higher the antimony percentage in the grid metal, the lower the charge voltage and conversely, as the antimony is decreased so the charge voltage increases until pure lead is attained, which produces the highest voltage on charge. Since the use of antimony has gradually been lowered from a maximum of about 12.0% to a maximum of about 6.0% antimony, the charge voltage of average batteries has increased.
Contaminant metals, hereinafter referred to as metal impurities including antimony from the positive grids, during service life, slowly goes into solution in the sulfuric acid electrolyte and from there it is believed to electroplate onto the surface of the negative plates. Once there, it acts as an additional electrode with the grid and the lead active material of the negative plates. This combination creates local action, promoting self-discharge and contributes to poor wet battery shelf life. In addition, the battery's charge voltage slowly decreases during life and, in the voltage regulated electrical circuit of a car, the difference between the two becomes greater. The car voltage regulator is set at a voltage just slightly higher than the normal charge voltage of the battery. Thus, the generator is able to restore electrical energy to the battery, as needed, to keep it charged. With metal deposition and the lowering of the battery charge voltage, the generator output into the battery increases as an overcharge, which hastens the deterioration of the battery in service, until failure occurs. Therefore, it is very desirable to inhibit the detrimental effects of antimony on the negative plate.